Gear transmission comprising two worms

ABSTRACT

The driving shaft (11) is equipped with two oppositely threaded worms (13a, 13b) with which respectively mesh on opposite sides two worm-wheels (14a, 14b) of a first kinematic channel and two worm-wheels (16a, 16b) of a second kinematic channel, and the worm-wheels of the two channels situated on the same side of the shaft (1) mesh together, the worm-wheels (14a, 14b) of one channel being constrained to rotate with intermediate gear-wheels (15a, 15b) which mesh with a gear-wheel (17) mounted on the driven output shaft, the shafts which carry the worm-wheels of the two kinematic channels being mounted to float in the axial direction.

The present invention concerns a gear transmission having a particularlyhigh efficiency, said transmission being suitable, inter alia, forconstituting a speed reducer suitable for driving automobile vehicleaccessories, for example.

There has already been proposed, in French Pat. No. 2 372 998, a geartransmission comprising a driving shaft and a driven shaft, withdifferent angular speeds, the two shafts having axes fixed relative toone another, one of the shafts carrying two worms having algebraicallydifferent pitches and being connected to the other shaft by twodifferent gear kinematic channels each comprising one of theaforementioned two worms; at least one of the two kinematic channelscomprises at least one intermediate worm-wheel having an axis which isfixed, neglecting tolerances, relative to the axes of the two shafts ofthe transmission. In a preferred embodiment of this gear transmission,it is proposed that the driving shaft of the transmission carry twoworms with opposite directions, each of the two kinematic channelscomprising a helical tooth intermediate worm-wheel meshing with one ofthe two worms, the two intermediate worm-wheels having their axes onopposite sides of the shaft carrying the two worms and each being fastwith a coaxial intermediate gear-wheel, these two intermediategear-wheels meshing with a common output gear-wheel fast with the drivenshaft; the axes of the driving and driven shafts are perpendicular andaxes of the intermediate worm-wheels are parallel to the axis of thedriven shaft.

A device of this kind gives total satisfaction, in particular when it isused as a speed reducer for driving windshield wipers of an automobilevehicle by means of an electric motor. The two kinematic channels of atransmission in accordance with the hereinabove defined embodimenttransmit in equal proportions the power fed into the driven shaft; eachworm on the driven shaft is subjected to reaction forces and, if thepitches of the worms are equal and opposite, these reactions aresymmetrical relative to the central point of the double worm: in a planepassing through the axis of the double worm and perpendicular to theaxes of the two intermediate worm-wheels, the reaction components areequal, opposite and colinear and consequently cancel one another out; onthe other hand, in a plane perpendicular to the preceding plane ofprojection and passing through the axis of the double worm, orperpendicular to this axis, the components of the reactions on the twoworms do not cancel out and there results a bending moment in the wormand thus reaction forces in the bearings supporting the shaft carryingthe double worm. The existence of this reaction force of the shaft onthe bearings which carry it explains why the efficiency of atransmission of this kind is approximately 70%, which could be regardedas highly satisfactory as compared with prior art transmissions.Moreover, the magnitude of the reactions on the driving shaft bearingsis directly proportional to the inclination of the helical threads ofthe double worm; thus in practice it has been necessary to limit theinclination of the threads of the double worm to a value in the vicinityof 15°; it is known, however, that the efficiency of transmissionbetween a worm and a tangential gear-wheel is optimum for values of thethread inclination two to three times higher. This practical limitationon the inclination of the worm threads thus also constitutes alimitation on the efficiency of a transmission of this kind.

The objective of the present invention is to define a gear transmissionof the type mentioned hereinabove, said transmission havingsignificantly improved efficiency. This improvement in efficiency isachieved by reducing or completely eliminating the torque produced bythe reaction forces exerted on the two worms of the transmission, whencethere results a considerable reduction or even elimination of thereaction forces exerted by the shaft carrying the worms on its bearings.For the same power transmission capability, it is thus possible toreduce the dimensions of the double worm and to employ smaller bearingsto support the shaft carrying the double worm. It is thus seen that, forthe same transmitted power, the weight and cost of the transmission maybe reduced as compared with a transmission of the type described inFrench Pat. No. 2 372 998. Moreover, the elimination of reaction forceson the bearings makes it feasible to consider increasing the inclinationof the worm threads, resulting in an increase in the overall efficiencyof the transmission.

In accordance with the invention, each of the two kinematic channels ofthe preferred prior art embodiment of transmission as described inoutline hereinabove is duplicated; thus each kinematic channel comprisesan intermediate worm-wheel linked to an intermediate gear-wheel, whichmeshes with the output gear-wheel, and an auxiliary worm-wheel, whichmeshes with the aforementioned intermediate worm-wheel and with that ofthe two worms which does not cooperate with said intermediateworm-wheel. Thus there is proposed a transmission which comprises twoworms, of opposite direction, and four worm-wheels. In each kinematicchannel, the intermediate worm-wheel cooperates with one of the wormsand the auxiliary worm-wheel with the other, the intermediate andauxiliary worm-wheels meshing with one another. In accordance with theinvention, it is further provided that the two intermediate worm-wheelsof the two kinematic channels are able to move parallel to their axes bysubstantially equal amounts in the same direction; the two intermediateworm-wheels are thus motionally linked and their median planeperpendicular to their axes may be moved away from the axis of thedouble worm to a greater or lesser extent whilst remaining parallelthereto; transverse displacement of the intermediate worm-wheels isunrestricted during operation of the transmission.

Adoption of the two characteristics of the invention as definedhereinabove makes it possible to achieve particularly beneficialresults. First of all, it is found that there is a self-balancing offorces between the two transmission kinematic channels. If one of thekinematic channels tends to transmit a higher force than the other,there results an increase in the reaction forces on the intermediate andauxiliary worm-wheels of the more heavily loaded kinematic channel andthe component of the reaction exerted on the intermediate worm-wheelparallel to its axis increases whereas the like component correspondingto the other intermediate worm-wheel decreases in a correlative manner.There results a displacement of the set of two intermediate worm-wheelsparallel to their axes, in a direction which tends to modify theaforementioned components until their values are equal and opposite. Theunrestricted displacement of the two intermediate worm-wheels thereforeprovides for continuous self-balancing of forces between the twokinematic channels.

Also, knowing that the two kinematic channels are equally loaded inoperation, it is possible to examine the operation of one only of thetwo kinematic channels, the position of the intermediate worm-wheelsrelative to the axis of the shaft carrying the worms being assumedfixed. The worm carrier shaft is mounted without longitudinal thrustbearings: if the power transmitted by the auxiliary worm-wheel is notstrictly equal to the power transmitted by the intermediate worm-wheelof the same kinematic channel, it is clear that the longitudinalcomponents of the reaction forces exerted on the two worms will nolonger be equal and that the worm carrier shaft will be displacedlongitudinally to secure equal values of the opposed longitudinalcomponents of the two reaction forces. Thus it is seen that there is anautomatic balancing of the power transmitted by the auxiliary worm-wheeland the intermediate worm-wheel of the same kinematic channel. Giventhat the two kinematic channels are equally loaded by virtue of theself-balancing effect of the unrestricted displacement of theintermediate worm-wheels, it is seen that the two kinematic channelsensure that the worm carrier shaft is always in the same longitudinalposition under equilibrium conditions.

In other words, it is seen that by virtue of the unrestrictedlongitudinal positioning of the worm carrier shaft and the unrestrictedtransverse positioning of the two intermediate worm-wheels relative tothe double worm, there is obtained a self-balancing effect whereby onequarter of the power is transmitted by each of the four worm-wheelsassociated with the double worm. As a result of this, the four reactionforces exerted on the double worm have equal values. If the arrangementis such that the plane of the axes of an intermediate worm-wheel and theauxiliary worm-wheel which does not mesh with it are perpendicular tothe axis of the double worm, the four reaction forces exerted on thedouble worm are distributed in such a way that their components in theplanes passing through the axis of the double worm and parallel orperpendicular to the axes of the worm-wheels are equal and opposite inpairs; the result of this is that there is no longer any bending momentwithin the double worm and that there is no longer any reaction force onthe worm carrier shaft bearings. A transmission mechanism of this kindhas the same efficiency irrespective of the rotation direction of thedouble worm.

On the other hand, if the plane of the axes of an intermediateworm-wheel and the auxiliary worm-wheel which does not mesh with it isslightly oblique relative to the axis of the double worm, there will notbe achieved exact compensation of the components of the four reactionforces and there will thereafter remain a very small bending moment inthe shaft and a small reaction force on the worm carrier shaft bearings.

It should be noted that the floating disposition of the two intermediateworm-wheels linked motionally provides a number of secondary advantages.

Firstly, if it is assumed that the output gear-wheel is disengaged fromthe two intermediate gear-wheels which drive it and if the worm carriershaft is assumed fixed, the mounting of the output gear-wheel on theintermediate gear-wheels is considerably facilitated by the possibilityof moving said intermediate worm-wheels transversely. Such transversedisplacement causes rotation in opposite directions of the twointermediate worm-wheels and the intermediate gear-wheels which areassociated with them and, for a particular value of this displacement,it becomes possible to mesh the output gear-wheel with the twointermediate gear-wheels. Note that the transverse position of theauxiliary worm-wheels has no effect with regard to the assembly oroperation of the transmission. If the two intermediate worm-wheels aremoved transversely while the auxiliary worm-wheels are fixed, thereoccurs helical displacement of the intermediate worm-wheel relative tothe auxiliary worm-wheel, without any other modification to the system.The position of the auxiliary worm-wheels may therefore be fixedrelative to the axis of the double worm; however, there is no reason whythe auxiliary worm-wheels should not move transversely relative to theaxis of the double worm at the same time as the intermediateworm-wheels, which leads to the idea of mounting the two intermediateworm-wheels and the two auxiliary worm-wheels on common transversedisplacement means.

Secondly, transverse displacement of the intermediate worm-wheelsrelative to the axis of the double worm may be exploited to render thetransmission irreversible when the latter is not functioning. If thetransmission is assumed to be halted and if it is assumed that the twointermediate worm-wheels are in their equilibrium position, it is clearthat the transmission in accordance with the invention, given its highefficiency, is necessarily reversible. It is often useful to procureirreversibility when stopped, however. The presence of transversedisplacement means for the intermediate worm-wheels in the device inaccordance with the invention provides for rendering the transmissionirreversible. If the transverse displacement means for the twointermediate worm-wheels is moved, all the conditions for correctoperation of the transmission are eliminated and conditions are createdleading to very poor efficiency, whence there results in practiceirreversibility of the transmission with regard to any input from theoutput gear-wheel. This transverse displacement of the intermediateworm-wheels may be achieved manually or by any appropriate mechanical,pneumatic, hydraulic or electromagnetic means. If the force applied tothe transverse displacement means for the intermediate worm-wheels isremoved and if the double worm is again driven in rotation, thetransverse displacement means automatically return to the position inwhich the reaction forces on the double worm balance out and thetransmission efficiency is optimized.

It is clear that the worms on the worm carrier shaft may be worms withone or more threads. The worm-wheels which mesh with these worms arecylindrical worm-wheels with helical teeth compatible with the twoworms. One of the auxiliary worm-wheels has a righthand helical threadwhereas the other has a lefthand helical thread; likewise, one of theintermediate worm-wheels has a righthand helical thread whereas theother has a lefthand helical thread. The intermediate gear-wheels andthe output gear-wheel preferably have straight teeth, but these teethmay nevertheless be very slightly helical, the helix angle then beingvery different from the angles adopted for the worms and the worm-wheelsof the transmission.

The object of the present invention is therefore the new industrialproduct which constitutes a gear transmission comprising a driving shaftand a driven shaft, with different angular speeds and substantiallyperpendicular axes, the two shafts having axes fixed relative to oneanother, one of the shafts carrying two worms of opposite direction,being linked to the other shaft by two different gear kinematic channelsand being unrestricted in terms of its longitudinal position, each ofthe two kinematic channels comprising a helical tooth intermediateworm-wheel meshing with one of the two worms, the two intermediateworm-wheels having their axes on opposite sides of the shaft carryingthe two worms and each being fast with a coaxial intermediategear-wheel, the two intermediate gear-wheels meshing with a commongear-wheel driving the transmission shaft, which does not carry theworms, the axes of the intermediate worm-wheels being substantiallyparallel to the axis of the shaft which does not carry the worms,characterized in that each kinematic channel is duplicated andcomprises, in addition to the aforementioned intermediate worm-wheel, ahelical tooth auxiliary worm-wheel meshing with that of the two wormswhich does not cooperate with said intermediate worm-wheel, saidauxiliary worm-wheel meshing with the intermediate worm-wheel of thesame kinematic channel, the two intermediate worm-wheels of the twokinematic channels being subject, in operation, to unrestricteddisplacement parallel to their axes of substantially the same magnitudeand in the same direction.

In a preferred embodiment, the two intermediate gear-wheels and thegear-wheel with which they both mesh have straight teeth; the pitches ofthe two worms are equal and opposite, the radii of the pitch cylindersof the two intermediate worm-wheels are equal to one another, as arethose of the two intermediate gear-wheels; the two intermediateworm-wheels are linked together by means able to move in translationparallel to the axes of the two intermediate worm-wheels. In a firstvariant, the means linking the two intermediate worm-wheels is a platerelative to which the two intermediate worm-wheels may rotate freely; ina second variant, the means linking the two intermediate worm-wheels isan idler roller which cooperates with a corresponding rolling trackformed on each of the two intermediate worm-wheels. Provision mayadvantageously be made for the means linking the two intermediateworm-wheels also to link the two auxiliary worm-wheels, which can thenrotate freely relative to this linking means.

In a first embodiment, the gear-wheel which meshes with the twointermediate gear-wheels has its pitch cylinder externally tangential tothe pitch cylinders of the two intermediate gear-wheels. In anotherembodiment, the gear-wheel which meshes with the two intermediategear-wheels has its pitch cylinder internally tangential to the pitchcylinders of the two intermediate gear-wheels.

Provision may also be made for the radii of the pitch cylinders of theauxiliary worm-wheels to be equal to the radii of the pitch cylinders ofthe intermediate worm-wheels; it is also possible for these radii to bedifferent, however.

Provision may advantageously be made for the axis of the intermediateworm-wheel of one kinematic channel and the axis of the auxiliaryworm-wheel of the other kinematic channel to define a planeperpendicular to the axis of the shaft carrying the two worms; however,it is also possible to provide for the plane defined by these twoworm-wheel axes to be oblique relative to the axis of the shaft carryingthe two worms.

In a preferred application of the transmission in accordance with theinvention, the driving shaft is that which carries the two worms, thetransmission being a speed reducer; the two worms on the driving shaftare disposed adjacently on said shaft, the driving shaft being supportedby two bearings disposed on the same side of the assembly formed by thetwo adjacent worms; the driving shaft is driven by an electric motor andthe driven shaft actuates an automobile vehicle windshield wiperlinkage.

To better explain the object of the invention, there will now bedescribed by way of purely illustrative and non-limiting examples anumber of embodiments shown in the accompanying drawing.

In this drawing:

FIGS. 1a and 1b schematically represent the distribution of reactionforces for a transmission in accordance with the invention comprising aring output gear-wheel, FIG. 1a showing the projection of the reactionforces on the plane passing through the axis of the double worm andperpendicular to the axes of the worm-wheels, whereas FIG. 1b shows theprojection of the reaction forces onto the plane passing through theaxis of the double worm and perpendicular to the preceding projectionplane;

FIG. 2 shows in plan view a motor-gearbox in which the gearbox partconsists of a transmission in accordance with the invention, the outputgear-wheel being externally tangential to the intermediate gear-wheels,the auxiliary and intermediate worm-wheels having the same pitchcylinder;

FIG. 3 represents a cross-section on the line III--III in FIG. 2;

FIG. 4 represents another embodiment of the intermediate worm-wheeltransverse displacement means for a transmission of the type shown inFIG. 2;

FIG. 5 represents a variant of the speed reducer of FIG. 2, in whichvariant the output gear-wheel is a ring gear-wheel internally tangentialto the intermediate gear-wheels;

FIG. 6 is a partial cross-section on the line VI--VI in FIG. 5;

FIG. 7 represents a schematic of an alternative embodiment of thetransmission in accordance with the invention in which the plane of theaxes of one auxiliary worm-wheel and of the intermediate worm-wheel ofthe other kinematic channel is not perpendicular to the axis of thedouble worm;

FIG. 8 schematically represents another alternative embodiment of thetransmission in accordance with the invention in which the diameters ofthe auxiliary worm-wheels are different from the diameters of theintermediate worm-wheels;

FIG. 9 schematically represents another variant of the transmission inaccordance with the invention in which the two characteristics of thevariants represented in FIGS. 7 and 8 are used in combination.

Referring to FIGS. 1a and 1b, the reference numeral 1 designates thedriving worm carrier shaft of a transmission in accordance with theinvention. The shaft 1 is supported by two bearings 2a, 2b in which itis free to slide. The shaft 1 carries two double-thread helical worms3a, 3b of equal and opposite pitch. The two worms 3a, 3b are disposed inthe area of the shaft 1 which is outside the area between the bearings2a and 2b. The gears of the transmission are represented in chain-dottedline. The transmission comprises two intermediate worm-wheels 4a, 4bfeaturing helical teeth compatible with the worms 3a, 3b; the twointermediate worm-wheels 4a, 4b have the same pitch cylinder; their axesare parallel to one another and perpendicular to the axis of the shaft1; they mesh with the worms 3a, 3b, respectively. The intermediateworm-wheels 4a, 4b are each fast with a straight tooth intermediategear-wheel, respectively designated 5a, 5b, the gear-wheels 5a, 5bhaving the same axes as the intermediate worm-wheels 4a, 4b,respectively. The transmission further comprises two auxiliaryworm-wheels 6a, 6b; the worm-wheels 6a, 6b have helical teeth and thesame pitch cylinders as the intermediate worm-wheels 4a, 4b; theworm-wheel 6a meshes with the worm 3a and with the intermediateworm-wheel 4b; the worm-wheel 6b meshes with the worm 3b and with theintermediate worm-wheel 4a; the axes of the auxiliary worm-wheels 6a and6b are parallel to the axes of the intermediate worm-wheels 4a and 4b;the planes of the axes of the worm-wheels 4a and 6a, on the one hand,and 4b and 6b, on the other hand, are perpendicular to the axis of theshaft 1. The intermediate gear-wheels 5a and 5b mesh with an outputgear-wheel 7 consisting of an internally toothed ring, the intermediategear-wheels 5a, 5b being internally tangential to the output gear-wheel7. In FIG. 1a R'₁, R'₂, R'₃ and R'₄ designate the projections onto theplane passing through the axis of the shaft 1 and perpendicular to theaxes of the worm-wheels 4a, 4b, 6a, 6b of the reaction forces exerted onthe shaft 1 by the intermediate worm-wheels 4a, 4b and the auxiliaryworm-wheels, 6a, 6b, respectively. In FIG. 1b, R"₁, R"₂, R"₃, R"₄designate the projections of the same reaction forces on the planepassing through the axis of the shaft 1 and parallel to the axes of theworm-wheels 4a, 4b, 6a, 6b. Although this is not shown in the drawing,the intermediate worm-wheels 4a, 4b are carried by common transversedisplacement means which enable them to move freely parallel to theiraxes, these displacements being of the same magnitude and in the samedirection. The output gear-wheel 7 is fast with the output shaft 8 ofthe transmission which is disposed coaxially with the output gear-wheel7.

The transmission which has just been described features two duplicatedkinematic channels: the first kinematic channel comprises theworm-wheels 4a and 6b; the second kinematic channel comprises theworm-wheels 4b and 6a. If the power transmitted by one of the twokinematic channels tends to exceed the power transmitted by the other,the components parallel to the axis of the worm-wheels of the forces R"₁and R"₄, on the one hand, and R"₂ and R"₃, on the other hand, becomeunequal and the resultant of these components is not zero; a resultantreaction force is thus exerted on the combination formed by the twointermediate worm-wheels 4a, 4b parallel to the axes of theseworm-wheels, which produces a transverse displacement of the combinationof the two worm-wheels in the direction which tends to cancel out saidresultant. It is thus seen that by virtue of the floating position ofthe worm-wheels 4a and 4b there is a self-balancing of the powerstransmitted by the two kinematic channels.

Moreover, in the same kinematic channel, the power transmitted from theshaft 1 to the shaft 8 is equally divided between the paths which passthrough the intermediate worm-wheel 4a and 4b and the auxiliaryworm-wheel 6b or 6a; if an imbalance tends to occur, the forces R'₁ andR'₄ or R'₂ and R'₃ become unequal and the resultant of these forces onthe axis of the shaft 1 becomes non-zero, which produces a longitudinaldisplacement of the shaft 1 in the direction which tends to cancel outthis resultant. It is thus seen that the unrestricted longitudinalpositioning of the shaft 1 provides for the self-balancing of the powerstransmitted by the two paths of the same kinematic channel.

In other words, the device which has just been described automaticallyensures equal distribution of the power transmitted from the double worm3a, 3b by the four worm-wheels 4a, 4b, 6a, 6b. As a result of this, thecomponents R'₁, R'₂, R'₃, R'₄, on the one hand, and R"₁, R"₂, R"₃, R"₄,on the other hand, are equal and symmetrical relative to the axis of theshaft 1. As a result, these components cancel out in pairs and the shaft1 is not subject to any bending moment, so that there is no reaction onthe bearings 2a and 2b of the shaft 1. Given these conditions, it isfound that the efficiency of a transmission of this kind isexceptionally high and may be as much as 80%. This transmission thusconstitutes a significant technical improvement relative to thedisclosure of French Pat. No. 2 372 998. The absence of reaction forceson the bearings 2a and 2b enables their dimensions to be reduced; theabsence of any bending moment in the shaft 1 enables the dimensions ofthe double worm to be reduced for a given transmitted power.

In FIG. 2 there is represented another embodiment of the transmission ofFIGS. 1a and 1b, said transmission being this time disposed in a casingand connected to an electric motor to constitute a motor-gearbox usable,for example, to drive the windshield wipers of an automobile vehicle.The components which have been described with reference to FIGS. 1a and1b and which bear reference numbers between 1 and 6 inclusive arerepeated identically in the embodiment of FIG. 2, where they aredesignated by reference numerals increased by 10 relative to those ofthe corresponding components in FIGS. 1a and 1b. The shaft 11 is fastwith the rotor 19 of an electric motor. The combination comprising theelectric motor and the transmission is disposed in a casing generallydesignatd 20, said casing carrying internally the field windings 19a ofthe electric motor. The worms 13a, 13b each cooperate with anintermediate worm-wheel 14a, 14b and an auxiliary worm-wheel 16a, 16b.The two intermediate worm-wheels 14a, 14b are fast with two intermediategear-wheels 15a, 15b with straight teeth which mesh with a straighttoothed output gear-wheel 17; the output gear-wheel 17 is externallytangential to the intermediate gear-wheels 15a and 15b and is fast withthe driven shaft of the transmission (not shown in the drawing).

In this embodiment, the four worm-wheels 14a, 14b, 16a, 16b are mountedbetween two plates 21 and 22; the worm-wheel 16b is carried by the plate21 in line with a notch in the plate 22. The plates 21 and 22 areparallel to one another and perpendicular to the axes of the worm-wheels14a, 14b, 16a, 16b whose shafts, respectively designated 24a, 24b, 26a,26b, are carried by the casing 20, as is clearly visible in FIG. 3. Theplates 21 and 22 slide freely on these shafts. The auxiliary worm-wheel16a is extended by a spacer sleeve 25, so as to have the same overallheight as the combination formed by an intermediate worm-wheel and theintermediate gear-wheel associated with it. The two plates 21 and 22bear on opposite sides of the set of worm-wheels and are linked togetherby fastening means not shown in the drawing. The combination of the twoplates 21 and 22 and the worm-wheels which they enclose thus constitutesa cage which is freely movable along transverse sliding shaftsconstituted by the four shafts 24a, 24b, 26a, 26b of the fourworm-wheels of the transmission. The self-balancing of the transmissionof power through the four worm-wheels is thus effected, in this variant,by virtue of the floating mounting of the cage 21, 22 and theunrestricted axial positioning of the shaft 11. The advantages obtainedwith this embodiment are the same as those indicated for the embodimentof FIGS. 1a and 1b.

Referring to FIG. 4, there is represented another embodiment of thetransverse displacement means for the intermediate worm-wheels of thetransmission of FIG. 2. To avoid all possibility of confusion, thecomponents in FIG. 2 which are also shown in FIG. 4 have been designatedby reference numbers increased by 100 relative to those which desginatethe corresponding components in FIG. 2. FIG. 4 is a cross-sectionthrough the axes of the intermediate worm-wheels of the transmission ofFIG. 2. In this variant embodiment, the auxiliary worm-wheels of thetransmission are not able to move transversely. The intermediateworm-wheels 114a, 114b are mounted on respective shafts 124a, 124b, saidshafts being held at the bottom of the casing 120 of the transmission.Below each intermediate worm-wheel 114a, 114b there is disposed anextender sleeve 127a, 127b in which is formed a respective groove 128a,128b. A plate 129 links the two intermediate worm-wheels 114a, 114b andencloses each worm-wheel at the level of the groove 128a, 128b, theworm-wheels being able to turn freely relative to the plate 129 butbeing fast with this plate by virtue of the cooperation of the platewith the grooves 128a, 128b. The plate 129 is fast with a sliding shaft130 able to move within a sleeve 131 fast with the casing 120, the axisof said sleeve being perpendicular to the axis of the double worm of thetransmission. Transverse displacement of the plate 129 is limited, onthe one hand, by its bearing on one end of the sleeve 131 and, on theother hand, by means of a washer 132 disposed at the bottom of thesliding shaft 130, externally of the sleeve 131.

In this variant of the embodiment of FIG. 2, only the intermediateworm-wheels are able to move transversely, but nevertheless all theadvantages of the embodiment respresented in FIGS. 2 and 3 are retained.

There is shown in FIGS. 5 and 6 another embodiment of the transmissionin accordance with the invention. In this embodiment, there are againand identically the double worm, the intermediate and auxiliaryworm-wheels and the intermediate gear-wheels as represented in FIG. 2.These common parts have been designated by reference numbers which areequal to those of the corresponding parts of the embodiment of FIG. 2increased by 200. Thus there is, within a casing 220, a shaft 211 whichcarries two opposite pitch helical worms 213a, 213b; these two wormscooperate with, on the one hand, two intermediate worm-wheels 214a, 214band, on the other hand, two auxiliary worm-wheels 216a, 216b, therelative positions of these worm-wheels being identical to those of theFIG. 2 embodiment. The intermediate worm-wheels 214a, 214b respectivelycarry straight toothed intermediate gear-wheels 215a, 215b which meshwith an output gear-wheel 217; however, in this embodiment the outputgear-wheel 217 is in the form of a ring which meshes internally with thetwo intermediate gear-wheels 215a, 215b, as already described inrelation to the embodiment of FIGS. 1a and 1b. The output shaft with thegear-wheel 217 is designated 218 and is disposed coaxially with thestraight-toothed gear-wheel 217. In this embodiment, the two auxiliaryworm-wheels 216a, 216b are not able to move transversely; only theintermediate worm-wheels 214a, 214b can move in translation parallel totheir axis by means of an arrangement which is represented in FIG. 6.

Each intermediate worm-wheel 214a, 214b comprises at its base arespective pulley-wheel 228a, 228b, the two pulley-wheels cooperatingwith a common roller 229 which enters the grooves in the twopulley-wheels 228a, 228b. The roller 229 comprises a shaft 230 which canturn freely in a sleeve 213 of the casing 220. The shaft 230 is able toslide freely within the sleeve 231 between, on the one hand, theposition in which the roller 229 bears against one end of the sleeve 231and, on the other hand, the position in which a washer 232 fast with theshaft 230 bears against the other end of the sleeve 231. Theintermediate worm-wheels and the intermediate gear-wheels with whichthey are fast may slide freely along their respective shafts, transversesliding nevertheless maintaining meshing between the intermediategear-wheels 215a, 215b and the output gear-wheel ring 217. Thisembodiment offers the same advantages as those previously described.

Finally, there are schematically represented in FIGS. 7 to 9 three otheralternative embodiments of the transmission in accordance with theinvention. In FIG. 7, it is seen that the shaft 301 rotates in twobearings 302a, 302b and carries, in the area not between these twobearings, two worms 303a, 303b with equal and opposite pitch. Thetransmission comprises two intermediate worm-wheels 314a, 314b fast withtwo intermediate gear-wheels 315a, 315b and two auxiliary worm-wheels316a, 316b; the four worm-wheels 314a, 314b, 316a, 316b have pitchcylinders of the same diameter and helical teeth. The two intermediategear-wheels 315a, 315b have the same diameter. The difference ascompared with all the embodiments previously described is that the planeof the axes of the two worm-wheels which cooperate with the same worm isnot perpendicular to the axis of the shaft 301 but slightly oblique. Inthis case, the reactions of the worm-wheels on the worms are not exactlysymmetrical, but these reaction forces nevertheless remain moderate ifthe offset X which creates the oblique relationship remains low. It willbe understood that, in order to conform to meshing constraints, X mustbe equal to or a multiple of the apparent pitch of the worm-wheels. Forgreater clarity, the output gear-wheel meshing with the intermediategear-wheels 315a and 315b has not been shown.

In FIG. 8 there is represented an embodiment in which the diameters ofthe pitch cylinders of the auxiliary worm-wheels are different from thediameters of the pitch cylinders of the intermediate worm-wheels;nevertheless, the axes of the worm-wheels cooperating with the same wormare in a plane perpendicular to the axis of the double worm. Thisembodiment provides for selecting auxiliary worm-wheels which aresmaller than the intermediate worm-wheels in order to reduce the overalldimensions of the transmission. Given that the components represented inFIG. 8 are similar to those represented in FIG. 7, the components ofFIG. 8 have been designated by reference numerals increased by 100relative to those of the corresponding components in FIG. 7.

The FIG. 9 variant corresponds to a combination of the FIG. 7 and 8variants. Given that the components represented in FIG. 9 are analogousto those represented in FIG. 8, on the drawing the components of FIG. 9are designated by reference numbers increased by 200 relative to thoseadopted for the corresponding components in FIG. 7. In this embodiment,the auxiliary worm-wheels 516a, 516b have a pitch diameter less than thepitch diameter of the intermediate worm-wheels 514a, 514b with whichthey mesh, and the planes of the axes of the worm-wheels 514a and 516a,on the one hand, and 514b and 516b, on the other hand, are slightlyoblique relative to the axis of the shaft 501. This arrangement providesfor reducing the overall dimensions of the transmission in the same wayas the arrangement of FIG. 8, but also makes it possible to arrange forthe plane tangential to the worm-wheels 514b and 516b situated at theend of the shaft 501 to be perpendicular to the axis of the shaft, whichcan be a favorable factor in reducing the overall dimensions of thetransmission. It will be understood that in this embodiment, as in thatof FIG. 7, the reaction forces on the bearings 502a, 502b are nottotally eliminated.

It will be understood that the embodiments described hereinabove are inno way limiting and can provide a starting point for all desirablemodifications without exceeding the scope of the invention; inparticular, the two intermediate gear-wheels may mesh externally on agear-wheel situated between them and centred on the line of the axes ofthe intermediate gear-wheels, this gear-wheel being fast with agear-wheel which meshes with the output gear-wheel, the shaft of whichconstitutues the output shaft.

I claim:
 1. Gear transmission with two kinematic channels comprising adriving shaft (1, 11, 211, 301, 401, 501) carrying two worms (3a, 3b;13a, 13b; 213a, 213b; 303a, 303b; 403a, 403b; 503a, 503b) with threadsinclined in opposite directions, each of the two kinematic channelscomprising two helical teeth intermediate worm-wheels (4a, 4b; 6a, 6b;14a, 14b; 16a, 16b; 214a, 214b; 216a, 216b; 314a, 314b; 316a, 316b;414a, 414b; 416a, 416b; 514a, 514b; 516a, 516b) each meshing with arespective one of the two worms on opposite sides, two of theseintermediate worm-wheels of the respective kinematic channels being fastwith a respective straight toothed intermediate gear-wheel (5a, 5b; 15a,15b; 115a, 115b; 215a, 215b; 315a, 315b; 415a, 415b; 515a, 515b) coaxialwith it, these two intermediate gear-wheels meshing with a commongear-wheel (7, 17, 217) keyed to a driven shaft, characterized in thatthe auxiliary worm-wheels of a given kinematic channel (4a, 6b-14a,16b-214a, 216b-314a, 316b-414a, 416b-514a, 516b and 4b, 6a-14b,16a-214b, 216a-314b, 316a-414b, 416a-514b, 516a) mesh together and arecarried by common displacement means enabling them to move parallel totheir axes in operation, whereas the shaft (1, 11, 211, 301, 401, 501)carrying the two worms is mounted so as to be free to move in thelongitudinal direction.
 2. Transmission according to claim 1characterized in that the pitches of the two worms are equal andopposite, the radii of the pitch cylinders of the two intermediateworm-wheels being equal to one another, as are those of the twointermediate gear-wheels.
 3. Transmission according to either of claims1 and 2, characterized in that the two intermediate worm-wheels arelinked together by means (21, 22; 129, 130; 229, 230) able to move intranslation parallel to the axes of the two intermediate worm-wheels. 4.Transmission according to claim 3 characterized in that the meanslinking the two intermediate worm-wheels is a plate (21, 22; 129)relative to which the two intermediate worm-wheels may freely turn. 5.Transmission according to claim 3 characterized in that the meanslinking the two intermediate worm-wheels is an idler roller (229) whichcooperates with a corresponding rolling track (228a, 228b) formed on therespective intermediate gear-wheel (214a, 214b).
 6. Transmissionaccording to either of claim 3 characterized in that the means (21, 22)linking the two intermediate worm-wheels (14a, 14b) also link the twoauxiliary worm-wheels (16a, 16b) which can rotate freely relative tosaid linking means.
 7. Transmission according to any one of claim 1characterized in that the gear-wheel (17) which meshes with the twointermediate gear-wheels (15a, 15b) has its pitch cylinder externallytangential to the pitch cylinders of the two intermediate gear-wheels.8. Transmission according to any one of claim 1 characterized in thatthe gear-wheel (7, 217) which meshes with the two intermediategear-wheels (5a, 5b; 215a, 215b) has its pitch cylinder internallytangential to the pitch cylinders of the two intermediate gear-wheels.9. Transmission according to claim 2 characterized in that the radii ofthe pitch cylinders of the auxiliary worm-wheels (6a, 6b; 16a, 16b;216a, 216b; 316a, 316b) are equal to the radii of the pitch cylinders ofthe intermediate worm-wheels (4a, 4b; 14a, 14b; 214a, 214b; 314a, 314b).10. Transmission according to claim 2 characterized in that the radii ofthe pitch cylinders of the auxiliary worm-wheels (416a, 416b; 516a,516b;) are different from the radii of the pitch cylinders of theintermediate worm-wheels (414a, 414b; 514a, 514b).
 11. Transmissionaccording to any one of claim 1 characterized in that the axes (24a,24b) of the intermediate worm-wheel of one kinematic channel and theaxis (26a, 26b) of the auxiliary worm-wheel of the other kinematicchannel define a plane perpendicular to the axis of the shaft (11)carrying the two worms.
 12. Transmission according to any one of claim 1characterized in that the axis of the intermediate worm-wheel (314a,314b; 514a, 514b) of one kinematic channel and the axis of the auxiliaryworm-wheel (316a, 316b; 156a, 516b) of the other kinematic channeldefine a plane which is oblique relative to the axis of the shaft (301,501) carrying the two worms.